The present invention relates to a method and apparatus for determining quality indicators for communication channels. In particular the present invention relates to determining whether individual channels of a communications system should be used for the communication of information.
In communication systems information is transferred over communication channels established on various types of communication links. The quality of these links affects the performance of the communication channels and the quality can be specified in a number of ways. One way to determine the quality of a communication channel, especially where information is digital and transferred in packets, is to take account of the number of successful and unsuccessful (failed) reception events for information carried along the communication channel. The exact manner of detecting successful or unsuccessful receptions can vary, but one way is to include error checking information with transmitted packets so that a determination may be made at the receiving side as to whether the received packets have been received correctly.
Communication links are likely to be subject to interference from time to time and intermittent reception failures will occur. Reception failures may be a particular problem where the communication link is a radio link, for example, in a cellular communication system between a mobile terminal and a fixed terminal.
Systems that use wireless links can often establish such links on a number of different system channels. If the system is a cellular system the links can be established with different radio end points. In such systems it is beneficial to monitor the quality of a wireless link that has already been established because if it is unsatisfactory, a different wireless link can be set up on a different system channel and/or to a different radio end point that will provide a superior quality wireless link and therefore a superior quality communications channel.
Intermittent reception failures are to be expected and although they are undesirable, they may not present any serious problems and are not necessarily an indication that the channel quality is poor. Conversely, periodic repeating reception failures or high levels of random reception failures are an indication that channel quality is inferior and may cause problems such as a reduction in communication channel bandwidth.
It is therefore necessary to monitor the quality of a communication link to obtain an indication of whether it is suitable for supporting a communication channel. The indication should account for the fact that a single packet failure by itself does not determine that the channel is of poor quality. However, periodic packet reception failures or high levels of random packet reception failures should be reflected in the indication because such failures do indicate a poor quality link.
In order to obtain an indication of this type, a history of link quality based on the number of communication channel packet reception successes and failures can be used to determine the link quality.
A simple approach for doing this is to use a counter which is incremented each time a packet is received correctly and decremented each time a packet is received incorrectly. A similar type of counter is mentioned in U.S. Pat. No. 5,271,011, which relates to a digital audio muting system for disabling the output of a data transmission system when a relatively high error rate is detected. The counter of the muting system is incremented when an error is detected and decremented by an amount at a programmable time interval. The counter may be incremented by a greater amount when more serious errors are detected. A threshold value can then be established for comparing to the value held in the counter. If the value held in the counter is larger than the threshold value, the quality can be deemed to be bad; if the value held in the counter is smaller than the threshold value, the quality can be deemed to be good.
However, this approach does not take account of the time at which packet reception fails. For example, if a regular periodic failure were to occur for every fifth packet transmitted, the quality of the communications channel is poor, due to interference of the communications link. Such a simple counter may not reflect this, even if a packet reception failure is given more weight than the packet reception success. In fact, for any counter which adds a first value for packet reception failure and subtracts a second value for packet reception success, the resulting value of the counter will decrease continuously (tend towards zero) in the presence of periodic or random errors, where the average error ratio is less than the first value divided by the sum of the first and second values. Similarly, the resulting value of the counter will increase continuously (tend towards the maximum value of the counter) in the presence of periodic or random errors, where the average error ratio is greater than the first value divided by the sum of the first and second values.
It is an object of the present invention to provide a mechanism for reliably evaluating the quality or average error ratio of a communications channel based on the number of packets that are transferred over the channel either successfully or unsuccessfully.
It is another object of the present invention that the mechanism should not place an undue burden on system resources in terms of processing or storage capacity requirements.
In accordance with the first aspect of the present invention there is provided apparatus for determining the quality of a communications channel carrying information units for reception, the apparatus having:
input means for receiving information identifying if correct or incorrect reception of transmissions on the communications channel occurred;
storage means for storing data indicative of the quality of the communications channel; and
update means coupled with the input means for performing a first update operation to update the stored data in the event of incorrect reception of information units and for performing a second update operation to update the stored data in the event of correct reception of information units,
wherein the stored data is updated to an extent dependent on the data stored by the storage means immediately before the occurrence of the event.
Because the stored data is updated to an extent dependent on the data stored before the event, the data stored in the updated form will reflect the history of the stored data. Because the history is reflected, incorrect reception events in the form of intermittent single packet failures will not dominate the stored data. However, a plurality of incorrect reception events in the form of repetitive periodic or high levels of random packet failures will begin to affect the stored data. Advantageously, because the history of the stored data is considered, more recent incorrect reception events can affect the stored data to a greater extent than incorrect reception events that occurred sometime ago. This means that the use of a quality history can be made to weigh more recent packet reception failures in favor of reception failures that occurred some time ago.
Preferably the stored data includes a value, the first update operation increases the stored value and the second update operation decreases the stored value, such that the value stored by the storage means increases or decreases with each operation by an amount dependent on the value stored immediately before the occurrence of the event. Alternatively, the first update operation may decrease the stored value and the second update operation increase the stored value, such that the value stored by the storage means decreases or increases with each operation by an amount dependent on the value stored immediately before the occurrence of the event.
The storage means may include a two byte counter for holding a value indicative of the quality of the communications channel. If the stored data includes a value and the first update operation increases the stored value for each event of incorrect reception, the counter may be incremented by a value obtained by subtracting the value that is held in the high byte of the counter immediately before the event from 128 decimal.
If the stored data includes the value and the second update operation decreases the stored value for each event of correct reception, the counter may be decremented by a value obtained by subtracting the value that is held in the high byte of the counter immediately before the event.
If the data does include a value, the quality of the communications channel may be determined by comparing the stored value of the storage means with a predetermined threshold value and determining the quality from the difference therebetween.
In accordance with a second aspect of the present invention there is provided a method for determining the quality of a communications channel carrying information units for reception, said method comprising the steps of:
monitoring if correct reception of information transferred by the communications channel occurred;
storing data indicative of the quality of the communications channel; and
performing a first update operation to update the stored data in the event of incorrect reception of information units otherwise performing a second update operation to update the stored data in the event of correct reception of information units,
wherein the stored data is updated to an extent dependent on the data stored immediately before the occurrence of the event.
Other aspects and optional features of the present invention appear in the appended claims, to which reference should now be made and the disclosure of which is incorporated herein by reference.